1. Field of the Invention
This invention pertains generally to excitation control systems for rotating electrical apparatus and, more particularly, to such systems employing rectifiers, such as three-phase semiconductor controlled rectifier (SCR) bridges, for converting alternating current (AC) to direct current (DC) to provide excitation voltage to a generator.
2. Background Information
Three-phase rectifier circuits are commonly employed to convert AC signals to DC signals. These circuits often use SCRs disposed in bridge segments, with typically one SCR for each polarity of each AC phase. Typically, a bridge firing control circuit controls the firing point for each rectifier in each AC cycle.
It is not uncommon for a plurality of SCR bridges to be operated in parallel with each of the corresponding bridge firing control circuits being controlled by a central firing control circuit. The central firing control circuit manages each of the bridge firing control circuits in order that the corresponding rectifiers in each of the parallel bridges conduct current at the same point in the AC waveform.
SCR bridges are commonly employed in an excitation control system to provide field excitation for a rotating electrical apparatus (e.g., large synchronous generators and motors, utility synchronous generators and motors, industrial synchronous motors and generators, synchronous generators and motors for naval or other shipping applications, synchronous generators and motors for oil well drilling rigs). For example, when the generator, is on-line, generator field excitation is provided thereto. However, certain faults in the output power circuit of the generator may disturb the voltage fed into the excitation control system, thereby causing the excitation to the generator to shut off.
For example, disturbances in the input voltage to the phase lock loop (PLL) of the control system may be caused by faults, such as single line-to-ground faults, line-to-line faults and three-phase faults that occur in the power circuit when the generator's circuit breaker contacts are closed. Accordingly, there is room for improvement.